There is a strong demand on picture contrast in video displays, but each display has a limited dynamic range. In general, the picture contrast cannot be increased by simply increasing the video signal amplitude because exceeding the display dynamic range causes unwanted effects. Additionally, newer flat panel displays like liquid crystal display (“LCD”) and plasma display panel (“PDP”) types can have a smaller dynamic range than the more traditional cathode ray tube (“CRT”) designs. However, an efficient use of display dynamic range depending on the picture contents can increase picture contrast and quality. Such dynamic contrast improvement (“DCI”) is disclosed in U.S. Pat. No. 6,285,413 B1 to Akbayir (“Akbayir”), which is incorporated herein by reference.
Akbayir teaches how to analyze a picture framewise and adjust the parameters of a dual segment transfer function depending on the analysis results for the best subjective picture quality. FIG. 1 is a graph of the dual segment transfer function disclosed in Akbayir. Each image frame is analyzed for four different characteristics: image average brightness, dark sample distribution, frame peak value, and signal to noise ratio. The transfer function consists of two segments (a lower segment for dark samples, and an upper segment for light samples) with an adaptive pivot point. Signal to noise ratio adapts the signal preparation (the noise estimator is not a part of DCI), whereas the image average brightness, dark sample distribution, and frame peak value control the transfer function. The gain of the lower segment is adaptive to the dark sample distribution. A higher gain results from fewer dark samples and a lower gain from a higher number of dark samples. The gain of the lower segment is limited to following the range:1.0≦Segment1_Gain≦Max_Gain1
The gain of the upper segment of the transfer function is adaptive to the frame peak value. It is computed such that a detected peak value lower than a nominal will be moved to the nominal peak value. If the detected peak value is equal to or higher than the nominal peak value then a gain of 1.0 is used (no change). The computed theoretical gain is limited then to a maximum value in order to avoid unnatural effects:1.0≦Segment2_Gain≦Max_Gain2
The third parameter of the transfer function is the pivot point. It is adaptive to the average image brightness and allows dark pictures to be made more contrasting and light. Low average brightness moves the pivot point to a lower level and high average brightness moves the pivot point to a higher level. The minimum pivot point value is about 7 IRE and the maximum about 40 IRE, where a 100 IRE (Institute of Radio Engineers units) luminance represents the white level of a standard video signal and a 0 IRE luminance represents the blanking level.
Akbayir teaches that the DCI transfer function is computed within the vertical blanking time (see Akbayir at column 8, lines 43–45). The vertical blanking time or vertical blanking interval (“VBI”) is the time required for the electron beam in a video CRT to be moved from the end of a frame back to the top of the screen. In a software-based video processing system, the microprocessor may need to perform many operations during the VBI. For example, VBI portions of television signals (“VBI signals”) can be used to transmit information other than traditional program video or audio, such as closed-caption text and stock market data. Performing DCI computations during the VBI reduces the amount of time that a microprocessor has to receive and process VBI signals and/or perform other operations during the VBI.
The present invention is directed to overcoming this problem.